If you clear cookies/history you may get multiple instances of plugincontainer.exe temporarily starting up and running (Use Task Manager to investigate/confirm that). If that is occurring it is a known issue, it should be fixed eventually, and should not result in a permanent hang. (This problem will still occur even if plugins are disabled {bug633427 } ).

Presumably it is not actually crashing (if it is then see [[firefox crashes]] )
Do you get any error messages ?
If you clear cookies/history you may get multiple instances of plugincontainer.exe temporarily starting up and running (Use Task Manager to investigate/confirm that). If that is occurring it is a known issue, it should be fixed eventually, and should not result in a permanent hang. (This problem will still occur even if plugins are disabled {bug633427 } ).
For more general info see [[firefox hangs]]
There is also an issue that causes problems when the bookmarks/history database is changed see http://blog.bonardo.net/2011/09/30/is-your-firefor-freezing-at-regular-intervals. Note
*the blog offers a workaround involving an add-on
*the issue is already fixed in Firefox 8 and 9